The present invention relates to an output circuit for CMOS VLSI, or more particularly, to a CMOS output circuit suitable for high-speed signal transmission between chips of a multi-chip module.
CMOS output circuits have a low output resistance value as compared with the characteristic impedance of a transmission line on a circuit board in order to improve driving capability. The CMOS input circuit has a high input impedance which generates a reflection noise on the transmission line. It is necessary to hold the received signals until this reflection noise disappears, thereby causing a great time delay in signal propagation. To avoid a circuit has been conceived which is inserted in the input circuit for absorbing an overshoot as disclosed in JP-A-No. 59-208771. If an undistorted transmission signal is to be obtained, however, matched termination of the transmission line is required. A conventional bipolar LSI is provided with a termination resistor at the receiving end of the transmission line to permit high-speed signal transmission. Since this system consumes a large amount of power in the termination resistor, however, application thereof to CMOS LSI would result in the loss of the greatest advantage that is a small power consumption of the CMOS circuit.
In a system with a termination resistor at the transmitting end of a transmission line, on the other hand, the packaging density is reduced if the termination resistor is mounted outside of the chip, thereby adversely affecting the high-integration advantage of the CMOS LSI. Although the termination resistor is desirably formed in the chip, this results in a great product variation of the element, thus posing the problem of matched termination.
International application No. WO 84/02620 published on July 5, 1984 discloses a circuit in which the reflection signal from the receiving end is prevented from being reflected at the transmitting end by rendering the output impedance of the output circuit substantially equal to the characteristic impedance of the transmission line. In the method therein the output impedance of the output circuit is substituted for the termination resistor. However, the production variations of the element causes output impedance variations of almost .+-.50%, and therefore this method is not satisfactory as a means for absorbing the reflection signal.